1. Field of the Invention
This invention relates to a lubricant oil composition, more particularly a composition efficiently preventing wear of diesel engines operating with large quantities of soot in their oil, and particularly suitable for diesel engines equipped with an exhaust gas recirculation (sometimes referred to as EGR) system.
2. Description of the Related Art
Lubricant oils have been used for internal combustion engines to lubricate internal combustion engines, devices in driving systems (e.g., automatic transmissions, shock absorbers and power steerings) and gears having sliding mechanical parts for their smooth operation. In particular, lubricant oil for combustion engines are used mainly for piston rings and cylinder liners, bearings for crank shafts and connecting rods, valve trains including cams and valve lifters, and other sliding mechanical parts. They are also used for cooling the engines, cleaning and dispersing combustion products, and prevention of rust and corrosion, in addition to the lubricating purposes. As described above, lubricant oils for internal combustion engines are required to exhibit a variety of function. These requirements are becoming even severer, as the engines become more functional produce higher power and are operated under severer conditions. The essential functions of a lubricant oil for internal combustion engines are to prevent wear and seizure by helping the engine operate smoothly under all conditions. Hydrodynamic lubrication prevails in an engine, but boundary lubrication may occur in some sections, e.g., valve trains and dead centers in the cylinders. In general, zinc dithiophosphate or the like is added to prevent wear in the boundary lubrication areas.
Recently, lubricant oils for internal combustion engines are required to have extended intervals between oil exchanging, to abate release of spent oils for environmental considerations. Association of European Automobile Manufacturers (ACEA) has already established the specifications involving extended intervals between oil exchanging for lubricant oils for internal combustion engines. The extended period is required also in Japan.
Air pollution by diesel engine exhaust gases, in particular by NOx, is becoming more acute worldwide. Therefore, many governments are strengthening restrictions on NO.sub.x and particulate matter exhausted from diesel engines, to abate these emissions. It is considered that the engine makers will have to equip diesel engines with an EGR system, which is already adopted for gasoline engines, to clear these restrictions which are becoming more and more stringent. An EGR system to abate NO.sub.x emissions will increase particulate matter emissions. It will increase soot in the diesel engine oil, which accelerates wear of the valve trains and pistons/cylinders, making it more difficult to extend service intervals. Improved engine combustion to abate particulate matter emissions leads to increased temperature at the piston, which accelerates deterioration of the lubricant by oxidation, increasing load on the oil.
The techniques to prevent wear of valve trans and pistons/cylinders by soot in the lubricant have failed to drastically extend oil service intervals in the midst of increased load on the oil.
A variety of techniques have been proposed to reduce wear of internal combustion engines; e.g., use of four types of additives including zinc dithiophosphate (Japanese Laid-open Patent Application No. 54-103404), combination of an organomolybdenum compound and zinc dithiophosphate (Japanese Laid-open Patent Application No. 54-113604), and combination of an organomolybdenum compound, salicylate and bis type succinimide (Japanese Laid-open Patent Application No. 5-230485). The techniques aimed at extended service interval of lubricant include a combination of an organomolybdenum compound, zinc dithiophosphate and polysulfide (Japanese Laid-open Patent Application No. 8-73878).
Unlike a gasoline engine, a diesel engine tends to suffer contamination of the engine oil with large quantities of soot produced as a result of incomplete combustion of diesel fuel, as described earlier. It is considered that the soot, having surface activity, adsorbs a polar additive present in the oil and scrapes off a film formed on a friction plane.
The conventional techniques to incorporate antiwear agents, such as zinc dithiophosphate, may not sufficiently prevent wear of diesel engines under severe friction/wear conditions in which the lubricant oil is contaminated with soot, the conditions much different from those associated with gasoline engines. As one of few techniques proposed so far to improve ability of diesel engines to prevent wear, a combination of molybdenum alkyl dithiophosphate (Mo concentration: 200 to 400 ppm), zinc primary alkyl dithiophosphate and salicylate is disclosed by Japanese Laid-open Patent Application No. 7-207290.
These techniques, however, fail to exhibit sufficient effects of preventing wear of diesel engines equipped with an EGR system, which provide lubricating conditions with large quantities of soot present in the engine oil. Therefore, the techniques to develop lubricant oil compositions for diesel engines which can prevent soot-induced wear of sliding members, e.g., valve trains and pistons/cylinders, have been strongly demanded.